Conventionally, as a device for transmitting the rotation of a pedal to a rear wheel of a bicycle, known is a toothed belt driving device containing a drive pulley connected to a rotational shaft of the pedal, a driven pulley connected to a rotational shaft of the rear wheel, and a toothed belt wrapped around these two pulleys. On an inner circumferential surface of the toothed belt are formed convex-shaped belt tooth portions with a predetermined pitch along a belt longitudinal direction. On each of the respective outer circumferential surfaces of the drive pulley and the driven pulley are formed pulley groove portions so as to mesh with the belt tooth portions.
In the case where the outer diameter of the driven pulley is less than that of the drive pulley, belt tension applied to each of the pulley groove portions of the driven pulley is greater than that of the drive pulley. For this reason, when belt tension is suddenly increased due to standing pedaling during riding or the like, and the belt extends partially, the jumping (tooth skipping) of the belt is likely to occur on the driven pulley. In particular, during riding in the rain, the driven pulley may get rain water, and the coefficient of friction between the pulley groove portions and the belt tooth portions decreases. Therefore, jumping is more likely to occur. In order to suppress the occurrence of the jumping of the belt on the driven pulley at the time when belt tension is suddenly increased during riding in the rain, in the toothed belt driving device disclosed in Patent Document 1, the shapes of the pulley groove portion of the driven pulley and the belt tooth portion of the toothed belt have been devised.